Microsatellite Analysis Indicates High Genetic Differentiation and Population Structure in Western Brook Lamprey in the Columbia River Basin

The western brook lamprey (Lampetra richardsoni) is a small non-parasitic lamprey that occurs along the Pacific coast of North America from Alaska to California.  This species resides entirely within fresh water during both its filter-feeding larval stage and its non-trophic adult phase.  In lampreys in general, downstream movement may occur during the larval phase and it is thought that brook lamprey adults undergo some compensatory upstream swimming prior to spawning.  However, such movement in the western brook lamprey is thought to be limited relative to that of the Pacific lamprey (Entosphenus tridentatus), which has a wide-ranging marine adult feeding phase and no apparent homing tendencies.  Recent analyses using high-resolution microsatellite loci indicate that there is little genetic differentiation among Pacific lamprey from central British Columbia to central California, but the life history of the western brook lamprey suggests that a greater level of population structuring might be present in this species.  Thus, we used eight microsatellite loci to assess population structure in the western brook lamprey at different geographic scales: 1) testing whether western brook lampreys from outside the Columbia River Basin (CRB) and separated from the CRB by salt water were genetically differentiated from western brook lampreys within the CRB; 2) determining the level of genetic differentiation among subbasins within the CRB, where each subbasin was distinct physically (at a minimum, separated by a portion of the Columbia River mainstem); and 3) determining the level of genetic differentiation within subbasins (where there were no obvious barriers other than distance; sampling sites within subbasins included first-order spawning tributaries that were a minimum of 16 river km apart).  To date, analyses have been conducted on approximately 35-40 western brook lampreys from each of 18 sites.  Preliminary results show high levels of genetic differentiation among western brook lamprey sampling locations and strong population structure, particularly at the two broadest scales examined (where the vast majority of pairwise F_ST values were significant and high, averaging approximately 0.3 and occasionally exceeding 0.7).  In Pacific lampreys from a much broader geographic range, F_ST values do not appear to exceed 0.1.  The F_ST values observed in western brook lampreys are also greater than those reported in many salmonid species for which population structure and local adaptation are widely recognized.